1. Field of the Invention
This invention relates to a primer composition adaptable for both steel sheet and plastic sheet as vehicle bodies.
2. Related Art Statement
Lately, the use of plastic materials for automotive parts has rapidly progressed since it can reduce the weight of the vehicle body, act as a rust-preventing means and increase freedom of design in the automobile industry. Moreover, the vehicle body is typically formed by intricately combining steel sheet with plastic sheet.
In this case, there has hitherto been adopted a method wherein the painting of all the materials is first performed and then the painted materials are assembled in a final step.
The steel sheet is degreased and phosphated to form a phosphate layer thereon. After an under coat is formed by electrodeposition, a chipping primer or a stoneger coat is partially applied and then an intermediate coat and a top coat are formed thereon.
On the other hand, the plastic sheet is degreased, dried and coated with a primer for the plastic sheet. Then, an exclusive intermediate coat and an exclusive top coat are successively formed thereon. In this case, the formation of the exclusive intermediate coat may be omitted.
Thereafter, the painted plastic sheet is assembled onto the painted steel sheet.
In another painting method, the plastic sheet is degreased, dried, coated with an exclusive primer, baked and then assembled onto the steel sheet which is previously degreased, phosphated and coated with an under coat, and thereafter the common intermediate coat and top coat are formed and baked on the assembly of the plastic sheet and steel sheet.
According to these conventional painting methods, however, the structure of the resulting paint coats formed is different between the plastic sheet and the steel sheet and the paint used and the painting step are also different, so that there are caused the following problems:
(1) Since the painting equipment must be separately arranged for the painting of steel sheet and for the painting of plastic sheet, not only the operation efficiency is poor but also the huge equipment investment is required;
(2) The difference in appearance such as color tone, gloss and the like is caused between the plastic sheet and the steel sheet to degrade the commercial value of the automobile as a whole;
(3) The difference in the durability of painted coating is caused between the plastic sheet and the steel sheet to detract from the appearance of the automobile over time; and
(4) When the plastic sheet is coated with the exclusive primer, assembled into the steel sheet and then the resulting assembly is subjected to a painting together, there problems (1)-(3) above are not caused. In the selection of paints for intermediate coat and top coat, however, when a soft coating is used in compliance with the plastic sheet, the weather resistance, polishing property, solvent resistance, stain resistance and the like are degraded, and when a hard coating is used in compliance with the steel sheet, the impact resistance, chipping resistance and the like on the plastic sheet are degraded.